ABSTRACT The fungal pathogen Cryptococcus neoformans is responsible for more than half a million deaths worldwide each year largely because of the AIDS pandemic. Epidemiological evidence indicates a high prevalence of non-symptomatic cryptococcal infections in the general population. Fatal systemic diseases often occur through re-activation of latent infections in patients when their immunity is impaired by HIV infection or immunosuppressive therapies. Although Cryptococcus typically grows in a haploid state, large polyploid Cryptococcus cells (mostly 4C and occasionally 8C) are observed in infected lungs and are proposed to promote cryptococcal latency and persistence. These polyploid cells can subsequently give rise to small haploid cryptococcal populations. It is unclear how ploidy reduction occurs in Cryptococcus during infection and whether this process contributes to Cryptococcus reactivation. Our preliminary data indicate that a subpopulation of Cryptococcus cells become large in serum with meiosis mutants showing increased proportion of cells with elevated cell size. Interestingly, we found that large polyploid cryptococcal cells induced by genotoxic stress showed meiosis-like features during de- polyploidization process in vitro. Consistently, blocking meiosis impedes ploidy reduction in vitro. The meiosis mutants also showed reduced fungal burden in the lungs compared to the wild type when polyploid cells were used to inoculate mice. Most excitingly, we found that the meiosis-specific recombinase gene DMC1 is activated in Cryptococcus during infection in the murine model of cryptococcosis. These preliminary data led us to hypothesize that Cryptococcus can respond to the hostile host environment by polyploidization and this fungus can return to normal ploidy through meiosis in a permissive host condition due to HIV infection, generating proliferative haploid progeny and causing fatal systemic infections. Thus the life-cycle associated progression of cryptococcosis might be similar to the gametogenesis process of cancer. In that process, various cancer cells become polyploid in response to genotoxic chemotherapy or radiation therapy. These polyploid tumor cells can de-polyploidize through a meiosis-like process, giving rise to rejuvenated and proliferative cancer cells with normal ploidy. In this exploratory R21 proposal, our objectives are to determine the occurrence of meiosis in Cryptococcus during infection (Aim 1), and to dissect the process of ploidy reduction in this pathogen and to assess the contribution of meiosis to cryptococcal pathogenesis (Aim 2). The proposed work is a stepping stone for further comprehensive investigation into the important and yet poorly understood aspects of cryptococcal disease progression (latency and reactivation).